U.S. Pat. No. 4,921,767, issued to Datta et al., on May 1, 1990, describes the basic method of manufacturing a luminescent screen for a color CRT by the (EPS) process, using dry-powdered, triboelectrically-charged screen structure materials that are serially deposited onto a suitable photoreceptor disposed on an interior surface of a faceplate panel.
In the EPS process described in the above-referenced patent, dry-powdered, triboelectrically charged, color-emitting phosphors are deposited on a suitably prepared, electrostatically chargeable photoreceptor. The photoreceptor comprises an organic photoconductive (OPC) layer overlying, preferably, an organic conductive (OC) layer, both of which are deposited, serially, on an interior surface of a CRT faceplate panel. The OC layer of the photoreceptor has a thickness of about 1 micron (.mu.m) and the overlying organic photoconductive (OPC) layer, preferably, has a thickness of about 5-6 .mu.m. Initially, the OPC layer of the photoreceptor is electrostatically charged to a positive potential, using a suitable corona discharge apparatus. Then, selected areas of the OPC layer are exposed to visible light to discharge those areas, without affecting the charge on the unexposed areas. Next, a first triboelectrically positively charged phosphor is deposited, by reversal development, onto the discharged areas of the photoreceptor to form phosphor lines of substantially uniform width and screen weight. The OPC layer is electrostatically recharged, re-exposed to visible light, and developed with second and third phosphor materials to complete the luminescent screen. Then, the phosphor materials are fixed, filmed and aluminized, as is known in the art. Finally, the screen is baked at a high temperature to volatilize the organic constituents of the screen as well as the OC and the OPC layers.
The OPC layer may be deposited by spin-coating, air-spraying, or electrostatically-spraying a suitable OPC solution onto the interior surface of the faceplate panel. A drawback of spin-coating is that various spin cycle speeds and orientations are required to obtain a substantially uniform coating. Also, the typical coating time for faceplate panel having a 51 cm diagonal dimension is about 90 seconds, and about 90% of the applied material is wasted. This process time is unacceptably long for a production environment in which an OPC application time of 8 seconds or less is desired. The material waste also increases the manufacturing cost of the CRT. A similar drawback is encountered when the OPC layer is air-sprayed onto the interior surface of the faceplate panel, using a conventional spraying apparatus. In addition, conventional air-spraying requires a multiplicity of passes across the interior surface to deposit an OPC layer having a thickness of 5-6 .mu.m, and large droplets of OPC material frequently are deposited onto the underlying OC layer, causing surface irregularities in the photoreceptor. These surface irregularities cause non-uniform electrostatic charging of the photoreceptor and corresponding non-uniformities in the luminescent screen. Electrostatic-spraying of the OPC solution to form the OPC layer is preferred, because it has none of the drawbacks of spin-coating or air-spraying and provides a substantially uniform OPC layer in about 8 seconds or less.
Regardless of the method of application of the OPC solution, it is necessary to determine, quickly and accurately, the actual thickness of the entire OPC layer, because its thickness is a critical process parameter in the manufacturing of a screen by the EPS process. OPC layer thickness determines the amount of corona charge that the layer can hold. If the OPC layer is too thin, not enough charge can be held to produce phosphor lines or dots of acceptable quality. If the layer is too thick, the high temperature bakeout step becomes more critical because of the excessive amount of organic material that must be removed from the screen prior to final tube assembly. Excessive OPC thickness also wastes valuable process index time during the application of the excess material.